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Write back VS write through on RAID controller

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Daynja

Weaksauce
Joined
Jan 1, 2005
Messages
119
Could someone explain the pros and cons for these 2 cache modes on a RAID controller?

Here's what I've gathered...

Write back just writes to the cache memory as fast as it can, and then writes it to the drive at the slower rate as the cache fills up. This is faster, but because data is sitting in the cache longer there is a chance of data loss if there is a crash.

Write through writes to the cache and then to the disk, and then makes sure the data is written to the disk (or something like that, I'm not really sure) This is slower, but safer.

I did some very rough tests by copying a 350MB file from one drive to the RAID 5 array...with write through it took about 25 seconds and with write back it took about 12 seconds.

This was with a promise fasttrack SX4000 with 256MB ECC cache...four 160GB drives in RAID 5.

The default option was write through and believe me, I did notice the slow write speeds. Should I change this to write back for faster speeds, or is it actually unsafe? How long does data sit in the cache? If it's a matter of a few seconds, then I really don't think that's a problem. Opinions ?
 
I'd recommend write back only if you have a battery back up on card, or for the entire system. If you lose power, nasty nasty data corruption.
 
defakto said:
I'd recommend write back only if you have a battery back up on card, or for the entire system. If you lose power, nasty nasty data corruption.
Click to expand...

how nasty? I do have a UPS, but I'd rather not have to rely on it. Nasty like all the data is screwed...or nasty like the current file being written is screwed?
 
I'm not sure of the level of nasty, but I would guess that it could be anywhere from total data corruption, to just what was in cache. I'd lean more towards worst case scenario as i'm pretty much a pessimist when it comes to computer hardware failing on any level.
 
Daynja said:
how nasty? I do have a UPS, but I'd rather not have to rely on it. Nasty like all the data is screwed...or nasty like the current file being written is screwed?
Click to expand...
UPS is not a replacement for battery backup for just the card. I'd get a bbu module before relying on write-back at all. If the UPS manages to keep the system up for a while, fine and good, but if there's new data being written (pretty likely!) during this interval, it will get lost->corrupted when the UPS gives up the ghost. The BBu will be able to hold out for much longer periods of time, or depending on the disk system, write out all the data after the system is off.
 
It's funny because the board actually has a spot for the battery backup for the card, but it is a cheaper model that doesn't have that feature.
 
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